This application is based on Japanese Patent Application Nos. 9-305148 (1997) filed Nov. 7, 1997, 9-305149 (1997) filed Nov. 7, 1997, 9-332587 (1997) filed Dec. 3, 1997, 10-98 (1998) filed Jan. 5, 1998, 10-1466 (1998) filed Jan. 7, 1998, 10-8236 filed Jan. 20, 1998, 10-192793 (1998) filed Jul. 8, 1998, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a refraction type semiconductor photo-detector, a semiconductor photo-detection device comprising the semiconductor photo-detector combined with an optical waveguide, and production methods thereof.
2. Description of the Related Art
Conventional refraction type semiconductor photo-detectors have a structure in which, as shown in FIG. 1A, a light incident angled facet 21a is formed across a photo-absorption layer 23a from the top surface, or, as shown in FIG. 1B, an angle begins from a layer right beneath a photo-absorption layer 23b while contacting with the photo-absorption layer 23b (for example, Japanese Patent Application 9-52760 (1997)).
In FIG. 1A, numeral 21a indicates a light incident facet, 22a is a p-InP layer, 23a is an InGaAs photo-absorption layer, 24a is an n-InP layer, 25a is an n-InP layer, 26a is a p electrode, and 27a is an n electrode.
Further, in FIG. 1B, numeral 21b indicates a light incident facet, 22b is a p-InP layer, 23b is an InGaAs photo-absorption layer, 24b is an n-InP layer, 25b is an n-InP substrate, 26b is a p electrode, and 27b is an n-electrode.
Still further, in FIGS. 1A and 1B, numeral 28 indicates an optical fiber as an example of an optical waveguide for conducting the incident light. This optical waveguide 28 is combined with a semiconductor photo-detector shown in FIG. 1A or 1B to construct a semiconductor photo-detection device.
In the production process of above described photo-detector, when forming the reverse-mesa optical incident facet by using wet etching with bromine-methanol or the like, in etching including the photo-absorption layer 23a as a narrow gap or where the photo-absorption layer 23a exists close to the etching, the photo-absorption layer 23a as a narrow-gap is relatively fast in etching speed and, since side etching is liable to occur, an etching irregularity such as uneven side etching tends to generate during deep etching, resulting in a problem of generating fine irregularities or waves on the etching surface.
When the spot size of incident light is large, effect of irregularities or waves is small. However, when an optical beam is focused and applied using a tapered fiber or a lens, this effect becomes conspicuous, the beam is diffused, and focusing of the beam is degraded.
Further, in the prior art structure, in order to obtain a high-speed response, the incident position must be set at the top surface side as possible so that the photo-absorption area is the smallest, when the incident light position is moved down to the substrate side, the photo-absorption part is required to be increased in length to make photo-absorption possible.
As a result, the photo-absorption area is increased resulting in degraded high-speed response characteristics.
Still further, in the above-described prior art semiconductor photo-detector, the chip is formed by using cleavage or the like from the vicinity of the light incident angled facet.
Thus, the chip does not have a guide structure for optical-connection with an optical waveguide such as an optical fiber.
At the time to connect the optical fiber with the photo-detector optically, when the optical beam center of the optical fiber is fitted to the center of the optical-absorption area of the photo-detector, the responsivity becomes maximum, and when the optical beam center of the optical fiber is shifted from the center of the optical-absorption area of the photo-detector, the optical-absorption amount of the photo-detector is decreased, thereby the responsivity is deteriorated.
Although permissible range of the shifting depends on size of the photo-detector and a direction of the shifting, etc., the permissible range is usually several xcexcm in the minimum direction.
As a result, in optical coupling with fibers or the like, fine mechanical adjustment of fiber optical end with an accuracy of several microns is required to a position where the responsivity is the maximum.
Therefore, there is a problem in that when fabricating a module (semiconductor photo-detection device) by combining a photo-detector with a fiber, a very precise positioning technique is necessary, and even a small deviation generates a degraded responsivity or a degraded response speed.
Therefore, in general, one or two lenses are inserted between the photo-detector and the fiber to moderate the positioning accuracy.
However, there is a problem in that the insertion of such a lens system leads to increases in the number of parts or fabrication steps resulting in an increase in module cost.
Further, there is a report of a structure in which to perform good optical coupling with the fiber without using the above lens system, the photo-detector is mounted on an optical fiber holding substrate having a V-shaped groove comprising silicon or the like. However, in this construction, it is required that the optical fiber holding substrate and the photo-detector be connected in high mechanical precision, which requires a high-precision positioning technique, and a small deviation generates reduction of responsivity or response speed.
Still further, even when the lens system is inserted as described above, there is some distance deviation between the device and the lens system during positioning, which is a cause of deviation in responsivity in a device with a small misalignment tolerance.
Yet further, in the above-described prior art semiconductor photo-detector, the electrodes 26a and 26b of the upper layer are, in general, alloyed with the semiconductor layer by heat treating metals such as AuZnNi for the case of p-type or AuGeNi for the case of n-type to form ohmic electrodes.
By virtue of such alloying, fine irregularities are generated between the electrodes 26a and 26b and the p-InP layers 22a and 22b as semiconductor, even if refracted light reaches here, it is diffuse reflected or absorbed by the electrode metal itself, and the electrode part is small in light reflectivity.
Therefore, although the thicknesses of the photo-absorption layers 23a and 23b can be reduced by increasing the effective absorption length through which the refracted light transits diagonally with respect to the layer thickness direction which is a characteristic of the refraction type semiconductor photo-detector, to obtain a sufficiently large responsivity, refracted light to the photo-absorption layers 23a and 23b is required to be sufficiently absorbed by one transit, there has been a limitation in reducing the thickness of the photo-absorption layers 23a and 23b. 
As a result, a transit time of carriers transitting the photo-absorption layers 23a and 23b is a limitation factor of response speed of the semiconductor photo-detector, and there is a problem in that an ultra-high speed and high responsivity device cannot be fabricated.
Yet further, in the prior art refraction type semiconductor photo-detection device, for example, as shown in FIG. 1A, the light incident facet 21a of the refraction type semiconductor photo-detector and an optical waveguide such as a single mode optical fiber are disposed in opposition, and a gas such as air or an inert gas is filled in between.
Here, since the gas has a refractive index of nearly 1 and the refractive index of the photo-detector material is constant, the refraction angle at the light incident facet 21a is determined only by the reverse-mesa angle.
In general, in the production process of refraction type semiconductor photo-detector, when fabrication is made by determining the reverse-mesa angle, mesa angles of devices in the same wafer become all in line with each other.
Since the refraction type semiconductor photo-detector utilizes an increase in effective absorption length by transiting light the optical absorption layer by refraction, the refraction angle is determined equally in the prior art, and the effective absorption length is also constant.
Therefore, there is a problem in that to change the effective absorption length according to various applications, wafers of different mesa angles or different absorption layer thicknesses are required to be prepared to change the refraction angle.
A first object of the present invention is, in a refraction type photo-detector, to provide a refraction type semiconductor photo-detector and a production method thereof which has a very flat angled light incident facet even to a fine size light beam.
A second object of the present invention is, in a refraction type photo-detector, to provide a semiconductor photo-detector, a semiconductor photo-detection device and production methods thereof which are capable of easily making high precision optical coupling when making optical coupling with fibers.
Further, a third object of the present invention is, in a refraction type photo-detector, to provide a semiconductor photo-detector which can provide a high responsivity even with a thin optical absorption layer and is capable of making ultra-high-speed operation.
Still further, a fourth object of the present invention is, in a semiconductor photo-detection device comprising a refraction type semiconductor photo-detector and an optical waveguide disposed in opposition to the photo-detector, to provide a semiconductor photo-detection device which is capable of making adjustment of responsivity according to the application using refraction type semiconductor photo-detectors of the same layer structure and the same mesa angles.
A semiconductor photo-detector according to Claim 1 of the present invention which attains the above object is characterized in that: a first semiconductor layer having a first conduction type, a second semiconductor layer having a second conduction type, and a photo-absorption part comprising a photo-absorption layer sandwiched between the first semiconductor layer and the second semiconductor layer are disposed on a substrate: at least the photo-absorption layer is formed at a position apart inside by a finite length from an end surface of the substrate; the end surface of the second semiconductor layer and the substrate or the end surface of the substrate is provided with a light incident facet angled inwardly as it separates from the surface of the second semiconductor or the surface of the substrate; and light incident to the light incident facet is refracted at the light incident facet and transits the photo-absorption layer diagonally with respect to the layer thickness direction.
Further, a production method of the semiconductor photo-detector according to Claim 2 of the present invention which attains the above object is characterized in that: a first semiconductor layer having an intrinsic or a first conduction type, a second semiconductor layer having the same first conduction type, and a growth layer comprising a photo-absorption part including a photo-absorption layer sandwiched between the first semiconductor layer and the second semiconductor layer are disposed on a substrate; a main inside part of the first semiconductor layer at the surface side, or the inside part and part of photo-absorption layer is converted selectively to a second conduction type by diffusion of an impurity; and an end surface of the substrate side growth layer except for the photo-absorption layer or the substrate is provided with a light incident facet angled inwardly as it separates from the surface side from a position apart by a finite length in a direction parallel to the substrate surface with respect to the photo-absorption part comprising the photo-absorption layer, whereby obtaining a semiconductor photo-detector in which incident light is refracted at the light incident facet and transits the photo-absorption layer diagonally with respect to the layer thickness direction.
Still further, a production method of the semiconductor photo-detector according to Claim 3 of the present invention which attains the above object is characterized in that: a first semiconductor layer having an intrinsic or a first conduction type, a second semiconductor layer having the same first conduction type, and a growth layer comprising a photo-absorption part including a photo-absorption layer sandwiched between the first semiconductor layer and the second semiconductor layer are disposed on a substrate; a main inside part of the first semiconductor layer at the surface side, or the inside part and part of photo-absorption layer is converted selectively to a second conduction type by ion implantation and subsequent anneal; an end surface of the substrate side growth layer except for the photo-absorption layer or the substrate is provided with a light incident facet angled inwardly as it separates from the surface side from a position apart by a finite length in a direction parallel to the substrate surface with respect to the photo-absorption part comprising the photo-absorption layer, whereby obtaining a semiconductor photo-detector in which incident light is refracted at the light incident facet and transits the photo-absorption layer diagonally with respect to the layer thickness direction.
Yet further, a semiconductor photo-detector according to Claim 4 of the present invention which attains the above object is characterized in that: a first conduction type semiconductor layer, a photo-absorption layer comprising an intrinsic or a first conduction type semiconductor layer, or a superlattice semiconductor layer or a multiple quantum well semiconductor layer, and a schottky electrode are disposed on a substrate; a semiconductor multilayered structure of large schottky-barrier height having a schottky barrier higher in schottky barrier height than the schottky barrier between the photo-absorption layer and the schottky electrode is formed between the photo-absorption layer and the schottky electrode; and an end surface of the substrate side growth layer except for the photo-absorption layer or the substrate is provided with a light incident facet angled inwardly as it separates from the surface side from a position apart by a finite length in a direction parallel to the substrate surface with respect to the photo-absorption part comprising the photo-absorption layer, wherein incident light is refracted at the light incident facet and transits the photo-absorption layer diagonally with respect to the layer thickness direction.
Yet further, a semiconductor photo-detector according to Claim 5 of the present invention which attains the above object is, based on the semiconductor photo-detector of Claim 4, wherein the semiconductor layer of large schottky-barrier height is In1xe2x88x92xxe2x88x92yGaxAlyAs (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61).
Yet further, a semiconductor photo-detector according to Claim 6 of the present invention which attains the above object is, based on the semiconductor photo-detector of Claim 4, wherein the semiconductor layer of large schottky-barrier height comprises In1xe2x88x92xxe2x88x92yGaxAlyAs (0xe2x89xa6xxe2x89xa61, 0xe2x89xa6yxe2x89xa61) and thin In1xe2x88x92uGauAs1xe2x88x92vPv (0xe2x89xa6uxe2x89xa61, 0xe2x89xa6vxe2x89xa61) disposed thereon.
Yet further, a semiconductor photo-detector according to Claim 7 of the present invention which attains the above object is, based on the semiconductor photo-detector of any one of Claims 4, 5 and 6, wherein a compositionally graded or step-graded layer from the same composition as the photo-absorption layer to the same composition as the semiconductor layer of large schottky-barrier height is disposed between the photo-absorption layer and the semiconductor layer of large schottky-barrier height.
The invention according to Claims 1 to 7 is characterized in that the light incident facet can be formed very flat and stable as compared with the prior art.
Here, since the light incident facet is formed on the substrate side growth layer except for the photo-absorption layer or on the substrate part from a position apart by a finite length with respect to the photo-absorption part comprising the photo-absorption layer, etching is prevented to the narrow-gap photo-absorption layer which is relatively higher in etching speed during formation of the light incident facet, generation of etching irregularity is almost eliminated, and a flat light incident facet can be formed with good yield.
Therefore, in the present invention, a narrow-gap photo-absorption layer is not included in the semiconductor layer forming the light incident facet, and the narrow-gap photo-absorption layer part does not contact the light incident facet, there is almost no generation of etching irregularity such as uneven side etching during deep etching, thereby obtaining a very flat light incident facet.
As a result, diffusion on the facet is prevented for a light beam focused using a tapered fiber or a lens, beam focusing is maintained, light can be absorbed by a small photo-absorption area, and an ultrafast photo-detector can be fabricated.
Further, fabrication of the photo-absorption part apart by a finite length from the light incident facet means that the photo-absorption part can be fabricated completely independent of the light incident facet, therefore, when light is incident by focusing with a lens or the like, the photo-absorption part of the device can be decreased in size to the same level as the beam size of the focus, thereby enabling ultrafast response.
Still further, since, even when the light incident position is moved down with respect to the top surface, the photo-absorption part can be set at an optimum position without increasing the photo-absorption area in consideration of refraction accordingly, and flexible construction is possible such as in hybrid integration on a silica-based lightwave circuit or the like.
Yet further, a semiconductor photo-detector according to Claim 8 of the present invention which attains the above object is characterized in that: a photo-absorption part comprising a semiconductor multilayer structure including a photo-absorption layer is provided on a substrate, an end surface is provided with a light incident facet angled inwardly as it separates from the surface side, and a V- or U-shaped groove is provided in opposition to the light incident facet, whereby light incident from an optical fiber disposed in the groove is refracted at the light incident facet and transits the photo-absorption layer diagonally with respect to the layer thickness direction.
Yet further, a production method of semiconductor photo-detector according to Claim 9 of the present invention which attains the above object is, based on the production method of semiconductor photo-detector according to Claim 8, wherein the light incident facet and the V- or U-shaped groove are formed simultaneously by etching.
Yet further, a semiconductor photo-detector according to Claim 10 of the present invention which attains the above object is, based on the semiconductor photo-detector according to Claim 8, wherein the light incident facet and its vicinity are buried in an organic substance.
Yet further, a production method of semiconductor photo-detection device according to Claim 11 of the present invention which attains the above object is fabricated, using the semiconductor photo-detector according to Claim 10, after making optical coupling with an optical waveguide, by removing the organic substance.
Yet further, a production method of semiconductor photo-detection device according to Claim 12 of the present invention which attains the above object is characterized in that, using the semiconductor photo-detector according to Claim 10, after making optical coupling with an optical waveguide, space between the semiconductor photo-detector and the optical waveguide is buried in with an organic substance.
In the invention according to Claims 8 to 12, the device has a groove in opposition to the light incident facet to be a fiber guide for conducting incident light, which part acts as a fiber guide, and high precision positioning is possible only by setting the fiber.
Further, by using a monolithic construction in which the present devices are arranged in parallel on a single chip, optical coupling is possible with multiple fibers collectively with high precision.
Still further, this device has a V- or U-shaped groove in opposition to the light incident facet, and the light incident facet part and its vicinity are buried in an organic substance.
Therefore, the V- or U-shaped groove part acts as a guide of optical waveguide such as a fiber or the like, and, since the light incident facet is protected with the organic substance, high precision optical coupling is possible by butting.
Yet further, by using a monolithic construction in which the present devices are arranged in parallel on a single chip, optical coupling is possible with multiple fibers collectively with high precision.
Yet further, a semiconductor photo-detector according to Claim 13 of the present invention which attains the above object is characterized in that: a photo-absorption part comprising a semiconductor multilayer structure including a photo-absorption layer is provided on a substrate; an end surface is provided with a light incident facet angled inwardly as it separates from the surface side, the substrate is protruded by a finite length from a tip part of the end surface, and light incident from an optical waveguide, precisely positioned by contacting against the protruded part of the substrate, is refracted at the light incident facet and transits the photo-absorption layer diagonally with respect to the layer thickness direction.
Since in the device of the invention according to Claim 13, part of substrate is protruded by a finite length from the tip of the light incident facet, this part acts as a stopper when a fiber is brought close from a far end in the optical axis direction, and the fiber tip will never contact against the important light incident facet to be damaged.
With the present invention, as compared with the prior art, optical coupling in the optical axis direction with a fiber or the like can be achieved precisely without delicate mechanical positioning.
Yet further, a semiconductor photo-detector according to Claim 14 of the present invention which attains the above object is characterized in that: a photo-absorption part comprising a semiconductor multilayer structure including a photo-absorption layer is provided on a substrate; an end surface is provided with a light incident facet angled inwardly as it separates from the surface side, a main reaching area of refracted incident light at the semiconductor layer above the photo-absorption layer is terminated with a substance having a smaller refractive index than the semiconductor layer, incident light is refracted at the light incident facet and transits the photo-absorption layer diagonally with respect to the layer thickness direction, and the transit light is total reflected by the substance of small refractive index on the semiconductor layer above the photo-absorption layer.
Since, in the device of the invention according to Claim 14, a main reaching area of refracted incident light at the semiconductor layer above the photo-absorption layer is terminated with a substance having a smaller refractive index than the semiconductor layer, light is completely total reflected on the upper surface, the refracted light transits two times the photo-absorption layer, and the effective absorption length is increased to two times.
Therefore, thickness of the photo-absorption layer to obtain a high responsivity can be considerably reduced.
Due to the remarkable decrease in the photo-absorption layer thickness, ultrafast operation of the device is possible while maintaining a high responsivity.
A semiconductor photo-detection device according to Claim 15 of the present invention which attains the above object is characterized by comprising a refraction type semiconductor photo-detector comprising a photo-absorption part including a semiconductor multilayer structure including a photo-absorption layer disposed on a substrate and an end surface provided with a light incident facet angled inwardly as it separates from the surface side, and an optical waveguide disposed in opposition to the device; space between the refraction type semiconductor photo-detection device and the optical waveguide is buried in a solid or liquid; whereby light incident to the light incident facet of the photo-detection device from the optical waveguide is refracted at the light incident facet and transits the photo-absorption layer diagonally with respect to the layer thickness direction.
Since in the semiconductor photo-detection device of the invention according to Claim 15, space between the refraction type semiconductor photo-detector and the optical waveguide is buried in a solid or liquid having a refractive index of greater than 1, by appropriately selecting the solid or liquid used to change the refractive index, it is possible to change the refraction angle on the photo-detector incident facet even when using a refraction type semiconductor photo-detector cut from the same wafer having the same layer structure and the same mesa angle construction, thus the responsivity can be adjusted according to the application.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.